As used herein, (poly-)isocyanates and (poly-)amines are understood to be mono-, di- and polyisocyanates or -amines.
It is known that in gas phase reactions, good mixing of the educts plays an important part in achieving high conversions and selectivities, above all when reacting polyfunctional reactants. An optimum and virtually spontaneous mixing of educts is decisive for the economy of continuous large-scale processes, when    a) the reaction of the educts is virtually spontaneous (high reaction speed),    b) one or more educts react with the product at comparably high reaction speeds to produce undesirable di- or oligomeric secondary products, and/or    c) the di- to oligomeric secondary products have a significantly higher boiling point than the educts or the desired product, condense in the reactor at reaction temperature and form deposits (e.g. cracker products, polymeric secondary products) on the reactor walls.
An example is the gas phase phosgenation of aromatic or (cyclo-)-aliphatic polyfunctional amines in a tube reactor. In the continuous process, the educts are normally introduced into a reactor in gas form, as disclosed in various patent applications (e.g. EP-A 699 657, EP-A 676 392, EP-A 593 334, EP-A 570 799, EP-A 289 840).
The reaction of phosgene with (poly)-amine to form (poly-)isocyanate is in competition with the secondary reaction of (poly-)amine and (poly-)isocyanate to form the corresponding urea oligomer. At the conventional reaction temperatures in the tube reactor the urea oligomers of gas phase phosgenation condense on the reactor wall. Improved mixing of the educts phosgene and (poly-)amine, whilst simultaneously avoiding back-flow caused by vortices in the tube reactor, increases the selectivity of (poly-)isocyanate formation and reduces the formation of urea. The quantities of condensation product in the tube reactor, which are deposited on the reactor wall, thus reducing the free tube cross-section and leading to a gradual increase in pressure in the reactor and finally determining the residence time of the process, are thereby reduced.
The reaction partners should be mixed within a time of up to 0.5 seconds to a degree of segregation of 10−3 The degree of segregation is a measure of the, incompleteness of mixing (EP-A 570 799).
The methods of realizing short mixing times are known in principle. Mixing apparatus with dynamic or static mixing devices is suitable. Static mixers are preferred. There are a number of different possible methods for the construction of static mixing organs, e.g. the use of nozzles, smooth-jet nozzles or Venturi nozzles known from combustion technology.
The disadvantages of many constructions are high pressure-loss or an arrangement that results in insufficiently rapid mixing or leads to re-mixing in the mixing zone or in the reaction chamber. High pressure loss in the mixing apparatus results in an increase in the amount of gaseous educt fed in. Higher pressure loss requires an increased boiling temperature to guarantee adequate pre-pressure. However, particularly with educts containing reactive functional groups, the increased boiling temperature causes thermal damage and therefore increases formation of by-products (yield/selectivity losses). In addition, insufficiently rapid mixing, or re-mixing, leads to an increased residence time of some of the educts and products and consequently to undesirable parallel or secondary reactions. Furthermore, insufficient mixing, particularly in strongly exothermic or endothermic reactions, causes an uneven temperature distribution in the reactor. These so-called “hot spots” and “cold spots” in the reactor result in increased thermal decomposition of the products or undesirably premature condensation of the products. Thermal decomposition products form a solid residue, which is deposited on the reactor walls. In this case the reactor is commonly fitted with an Inliner (reaction tube), which can be changed when it becomes encrusted, thus facilitating reactor cleaning. In the case of a reactor in the form of a cylindrical tube, for example, a simple cylindrically-rolled thin steel sheet of a resistant material is suitable as an Inliner.
The known disadvantages can be minimized if a single, individual nozzle of precisely-specified dimensions is used as the mixing device, which is fitted coaxially into a tube. The tube reactor then has a central nozzle and an annular space between the central nozzle and the wall of the tube reactor. The nozzle thus opens immediately into the reaction chamber (FIG. 1). The educts are mixed immediately after the nozzle outlet. One gaseous educt E1 (phosgene or (poly-) amine) is fed through the central nozzle, the other gaseous educt ((poly-)amine or phosgene) through the annular space between the central nozzle and the wall of the tube reactor into the reaction chamber. In this way, the stream of educt E1 is introduced centrally into the stream of educt E2 and mixed there. The flow rate of E1 must however be greater than the flow rate of E2. Nevertheless, the reactor lifetime that can be achieved with such an arrangement is still not entirely satisfactory.